1. Field of the Invention
The present invention relates to an image forming apparatus comprising an optical unit that consists of optical instruments for image formation mounted in a solidly shaped chassis.
2. Description of the Related Art
The image forming apparatuses of electrophotographic system (such as copy machines, printers, and facsimiles) generally write an electrostatic latent image by scanning the beam light on the surface of the electrically-charged photoreceptor, or by exposing the surface of the photoreceptor by means of LED array light source that is arranged in the direction of the rotational axis of the drum-shaped photoreceptor. The beam light is emitted from the light source, such as a laser diode, then scanned in the prescribed main scanning direction by a light scanning apparatus that comprises optical instruments, such as one or a plurality of reflecting mirrors (deflecting mirror), lenses (such as f θ lenses), polygon mirrors (rotational polygon mirrors), thereby being led to the photoreceptor drum.
Here, the deterioration of image quality, such as unevenness or warps of the image, might occur when the accuracy of irradiation position of the beam light onto the photoreceptor drum is low.
Particularly, in regard to the image forming apparatus of the tandem system (that is to say, color image forming apparatus), which forms respective toner images of a plurality of colors (for example, the four colors including Black, Magenta, Yellow, and Cyan) onto a plurality of photoreceptor drums independently then superimposes and transfers these images, when the accuracy of relative irradiation positions between each of beam lights corresponding to each color (especially, the accuracy of skew) is low, there might occur relative misalignment in transferring positions of toner images of each color in addition to the unevenness and warps of images, thereby distinctly deteriorating image quality.
Considering the above, by unitizing the optical instruments for image formation by means of mounting them in a solidly shaped chassis, improvement of the positioning accuracy between each of the optical instruments is achieved, and therefore, the structure in which such unit is mounted in the main body of the image forming apparatus is frequently applied. Hereinafter, the chassis constituting such unit is referred to as “unit chassis”. Here, above solid shape includes many shaping process such as solid forming, solid casting, solid molding, and solid pressing.
Generally, the unit chassis is fixedly supported at four places with respect to the supporting member in the side of the main body of the image forming apparatus. In this case, the gap between the supporting member and the unit chassis at the supporting member in one place may occur caused by, such as unevenness of the heights of supporting members in the side of the main body in four places and fluctuation in the shape of the unit chassis. This causes the warps of the unit chassis, thereby deteriorating the positioning accuracy of the optical instruments mounted therein.
In order to overcome the above, when the unit chassis is fixedly supported in three places against the supporting members in the side of the main body, the occurrence of such warps can be prevented, and the deterioration of the positioning accuracy of the optical instruments caused by the warp occurred by fixing the unit chassis can therefore be prevented.
Additionally, disclosed in Japanese Unexamined Patent Publication No. 1994-123849 is the structure, wherein warps of a base of the scanning optical apparatus is prevented by contacting a ball with the base of the scanning optical apparatus, then pressing the ball with a spring.
However, the unit chassis is nearly a cuboid in general. Therefore, when fixedly supported in three places, there may occur a part with weak restrained condition of displacement (hereinafter referred to as “free part”) outside of a triangle formed by fixedly supporting members in the three places. When the percentage of this free part becomes large, it easily resonates with vibration of the driving members of, such as polygon mirrors and peripheral devices mounted in the unit chassis, and therefore, with such resonance, deterioration of the positioning accuracy of the optical instruments has been occurred.
In addition, with fixedly supporting the unit chassis in mainly three places, it may further be fixedly and subsidiarily supported to the extent that the unit chassis may not be warped by means of such as the spring mounted in a holder as disclosed in Japanese Unexamined Patent Publication No. 1994-123849. However, this structure required additional subsidiary parts such as a spring that forms the fixedly supporting mechanism, as well as additional mounting processes thereof.
Consequently, this invention has been invented considering the foregoing conditions, and the purpose of this invention is to provide the image forming apparatus having the structure that is able to combine the preventions of warps and resonance of the unit chassis in which the optical instruments for image formation are mounted, neither with additional parts nor additional mounting processes thereof.